Appearance

Molt pattern is complex and nearly identical to that of Least Tern’s Eurasian counterparts the Little Tern and Saunder’s Tern (Olsen and Larsson 1995). During Prebasic molt, inner primaries are replaced 3 times, each wave of primary molt beginning at P1. First wave of primary molt begins during breeding season; second and third waves begin during midwinter and early spring, respectively. All primary molt arrested by start of breeding season, leaving 3 generations of feathers in place at this time, the oldest feathers being the outermost. This pattern of primary molt is unusual among Western Palearctic Sternidae but is also shared by Little and Saunder’s terns, Roseate Tern (S. dougallii) and some marsh terns (Chlidonias; Cramp 1985). Displays portlandica and pikei variations of Alternate I plumage (see below). The following descriptions of molts and plumages follows Massey and Atwood (1978) and Olsen and Larsson (1995), as supplemented by Atwood and Massey (1982), Thompson and Slack (1983), and Cramp (1985). Reader is referred to those treatments for detailed understanding of molts and plumages.

Hatchlings

Recently hatched chicks precocial and completely downy, ranging in color from nearly white to more usually tan or buff. Down usually spotted, speckled, or streaked with black, occasionally unmarked. Progressive appearance of Juvenal feathers during first 3 wk produces mottled, scaly appearance. Lacks black or brown throat usually found in Common and Arctic Tern chicks.

Juvenal Plumage

No detailed information about duration and order of feather emergence for Prejuvenal molt. Individuals in Juvenal plumage begin to appear near U.S. breeding colonies in Jun. Considerably brown and buff on back, wings, and head; pale buff or whitish feather edgings give back and wings a markedly scaly appearance. Crown is buff with variable dull black speckling that diminishes with age; lores buff or buffy white; broad, poorly defined, dull black eye-stripe extends variably toward nape; back, scapulars, and tertials dull grayish buff with darker subterminal brownish gray crescents and pale buff edges; rump pale gray, feathers finely edged with buff; tail shorter and less deeply forked than that of Definitive Alternate plumage of adult; rectrices bluntly pointed, mostly pale gray, narrowly edged with white and darkening toward tips to form dull grayish brown spots; inner primaries and alula dark grayish, narrowly edged with buffy white; outer 3 primaries slightly darker grayish, broadly margined with white on inner web except near tips; marginal wing-coverts dark, forming dark bar along leading edge of wing; greater, median, and lesser wing-coverts gray, edged with buff or dull white; chin, throat, sides of head below blackish occipital line, and upper breast, are white suffused with varying amounts of buff; lower breast, belly, undertail-coverts, sides, and wing-linings white.

Basic I Plumage

Prebasic I molt complete; begins during the fall after most juveniles have left breeding areas in U.S.; thus full Basic I plumage rarely observed in this area. However, some older fledglings show early stages of this plumage in late Jul and Aug while still in U.S. Prebasic I molt has not been studied in Least Tern, but this molt is probably similar to that of Little Tern (Olsen and Larsson 1995). In Little Tern, this molt begins on mantle and scapulars, followed by head, tertials, remaining upperparts, wing-coverts, and underparts. Molt begins in early Aug–late Sep, and Basic I plumage complete (except for remiges, rectrices, and some wing-coverts) by late Aug–early Nov. Primary and rectrix molt begins with P1 and R1, respectively; late Sep–early Dec. First wave of primary molt completed Apr–May; second wave of primary molt, starting again at P1, begins Feb–Mar. Some birds begin third wave, again starting at P1, in May. All waves of primary molt usually arrested late May–Jun (Cramp 1985). Continual replacement of primaries throughout winter and spring (completely overlapping Prealternate molt) is unusual, and assignment of all primary replacement to Prebasic molt is provisional. In general, buffy and brownish tones of back and wings are gradually replaced by gray; buffy regions of head become increasingly white, and scaly appearance of wings and back becomes less pronounced through this molt.

Forehead white, crown whitish gray with very limited black flecking; lores mainly white with black eye-line extending only partly toward base of bill; broad black stripe through eye extending across nape to form distinct nuchal collar; back, scapulars, tertials, and rump pale gray; wing-coverts pale gray and lacking buff or whitish edgings; marginal wing-coverts same color and pattern as in Juvenal plumage (see above), uncertain whether these are retained Juvenal feathers or new Basic I feathers that have same color and pattern; chin, throat, breast, belly, undertail-coverts, sides, and wing-linings white. Replacement of Juvenal remiges and rectrices variable. Dusky spots near tips of rectrices (best seen from above) are diagnostic of this plumage as compared to Alternate I and all subsequent plumages.

Alternate I Plumage

Pre-Alternate I molt partial; occurs late Feb–early Apr. This molt considered to be very similar to that of Little Tern (Olsen and Larsson 1995). In Little Tern, Prealternate I molt occurs late Feb–early Apr and involves head, body, wing-coverts, and rectrices, although rectrix molt may be continuation of Prebasic I molt (Cramp 1985).

In Least Tern, this molt has been shown to result in an Alternate I plumage that is variable in appearance. In s. California and Massachusetts, individuals banded as chicks that visited breeding colonies at 1 yr of age exhibited 2 distinctive plumages. Most 1-yr-olds exhibited a plumage almost identical to Basic I, while smaller percentage exhibited a plumage intermediate between Basic I and Definitive Alternate (Massey and Atwood 1978, Atwood and Massey 1982). These plumages have been referred to as the portlandica and pikei plumages, respectively, in this species by Atwood and Massey (1982). Other species of terns also show 2 distinctive subadult Alternate plumages, and it was originally believed that these represented second-year (portlandica) and third-year (pikei) Alternate plumages (see discussion in Massey and Atwood 1978). In study of s. California and Massachusetts Least Terns, all individuals exhibiting portlandica or pikei plumages that were of known age were 1 yr old, and no individuals known to be 2 yr old ever exhibited either of these plumages. Thus the portlandica or pikei “plumages” both appear to represent Alternate I plumage in this species (Atwood and Massey 1982).

Under most field conditions, portlandica plumage may be indistinguishable from Basic I plumage; however, latter never fully acquired during breeding season, whereas portlandica plumage may be seen throughout spring and summer. Any Least Tern observed near U.S. breeding grounds and characterized by a well-defined black eye-stripe, dark bar on leading edge of wing, dark gray inner primaries, black bill, and absence of scaliness or buffy coloration in back or wings may be safely assumed to be in portlandica plumage. Birds in pikei plumage have variably extensive blackish crown pattern usually approaching size and shape of full nuptial plumage, but strongly interspersed with white feathers; bill is yellowish. Presence of mostly white lores in pikei plumage reduces possible confusion during late summer with breeding birds that are beginning to molt. White flecking in anterior portion of black cap appears to be early stage in acquisition of winter (Definitive Basic) plumage in breeding adults. Wing characteristics of pikei birds somewhat variable; most have dark grayish bar on leading edge of wing and dark gray inner primaries characteristic of portlandica plumage. In the fall and away from breeding grounds, full Basic I plumage may be distinguished from portlandica plumage only by absence of dusky, subterminal U-shaped bar or spots on rectrices of the latter.

Definitive Basic Plumage

In Little Tern, Definitive Prebasic molt is complete; begins during breeding season (while adults are feeding young) with replacement of P1. Primaries replaced in order from innermost to outermost. Primary molt suspended during migration and completed on wintering grounds. Before last (outermost primaries) have been replaced, second wave of primary molt begins again at P1 during winter. A third wave of primary molt may begin during spring while second wave is still in progress. Second and third waves are arrested before start of incubation, and before outermost 2 (rarely 3 or 4) feathers have been replaced by second wave of molt. Thus, outermost feathers are 5–7 mo old at beginning of breeding season and are very dark from wear. Replacement of rectrices begins during summer, about when P1 is fully grown. Rectrices replaced from innermost to outermost, but R6 replaced with R4 (i.e., before R5). Rectrix molt suspended during migration and completed by midwinter. Second wave of rectrix molt may begin during late winter or early spring. Body molt begins midsummer on crown and upperparts; body molt suspended during migration and completed on wintering grounds (Cramp 1985). This molt considered similar in Least Tern (Olsen and Larsson 1995). Adult Least Terns also molt P1–P6 while nesting (Thompson and Slack 1983), but may begin this molt earlier (late May in s. U.S.; Chandler and Wilds 1994).

Definitive Basic plumage, probably never fully acquired within the species’ U.S. breeding range, likely resembles Alternate I (portlandica) plumage; but appearance of winter individuals of North American populations remains poorly known. Forehead, lores, and crown white with variable black flecking. Narrow black mask from nape through eye-coverts is broadest at nape, which is fringed with white. Upperparts gray, much like Alternate plumage, but dark lesser wing-coverts form bar on wing. Underparts white. Color and contrast in primaries depends on molt sequence and wear during Sep-Feb.

Definitive Alternate Plumage

Definitive Prealternate molt partial; occurs Feb–Apr; includes head, body, wing-coverts, and some tail-feathers. Familiar nuptial plumage, acquired at 2 yr old (feather homology unknown for those not in this plumage by 2 yr). Characterized by glossy black cap and nape with sharply defined white forehead area extending backward on sides of crown to above eye; black of crown extends forward through eye and lores to base of upper mandible; pale gray wings with usually 2–3 black outer primaries that have white edge on inner web. Back, scapulars, tertials, and rump pale gray (light bluish gray); outer rectrices elongate and pointed, inner rectrices shorter and bluntly pointed; outermost rectrix mainly white, with some graying toward tip on inner web; remainder of rectrices uniform pale gray, same color as back and rump; chin, throat, sides of head below eye, breast, belly, undertail-coverts, sides, wing-linings, and undersurface of rectrices white.

Bill And Gape

During breeding season, bill yellow with variable amount of black at tip (some individuals have completely yellow bill). Outside breeding season and when not in full Definitive Alternate plumage, bill variably dusky, blackish, or dull yellow orange. Gape orange in chicks; yellowish orange to orange in breeding adults.

Iris

Dark brown.

Legs And Feet

Reddish orange in adults during breeding season; blackish orange or dull black remainder of year. Legs of juveniles black to pink, grayish orange, or pale-spotted (Massey and Atwood 1978).